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Metabolism of Proline, Glutamate, and Ornithine in Proline Mutant Root Tips of Zea mays (L.)

Friedrich Miescher-Institut, Ch-4002 Basel, Switzerland

In excised pro_1-1 mutant and corresponding normal type roots of Zea mays L. the uptake and interconversion of [¹⁴C]proline, [¹⁴C]glutamic acid, [¹⁴C]glutamine, and [¹⁴C]ornithine and their utilization for protein synthesis was measured with the intention of finding an explanation for the proline requirement of the mutant. Uptake of these four amino acids, with the exception of proline, was the same in mutant and normal roots, but utilization differed. Higher than normal utilization rates for proline and glutamic acid were noted in mutant roots leading to increased CO₂ production, free amino acid interconversion, and protein synthesis. Proline was synthesized from either glutamic acid (or glutamine) or ornithine in both mutant and normal roots; it did not accumulate but rather was used for protein synthesis. Ornithine was not a good precursor for proline in either system, but was preferentially converted to arginine and glutamine, particularly in mutant roots. The pro_1-1 mutant was thus not deficient in its ability to make proline. Based on these findings, and on the fact that ornithine, arginine, glutamic acid and aspartic acid are elevated as free amino acids in mutant roots, it is suggested that in the pro_1-1 mutant proline catabolism prevails over proline synthesis.

² Recipient of a short term fellowship from European Molecular Biology Organization. Present address: Istituto di Genetica, Universitá di Milano Via Celoria, 10 I-20133 Milano, Italy.